In oil and gas well completion operations, frac or bridge plugs are necessary for zonal isolation and multi-zone hydraulic fracturing processes. The advantages of frac and bridge plugs made primarily from composite materials is well established since these products significantly reduce drill-out (removal) time compared to drill-out time for all metallic frac and bridge plugs. However, as drilling for oil and gas extends deeper underground and/or hydraulic fracking pressures increase, composite frac and bridge plugs are subject to higher pressures and operating temperatures. Additionally, since frac and bridge plugs are expendable items for short term use, it is important to manufacture these products at the lowest cost possible yet still meet performance requirements.
With higher pressures and operating temperatures associated with deep well fracking operations, increased stresses can be expected on frac and bridge plug products. Current composite frac and bridge plugs are made of fiberglass or carbon fiber with an epoxy resin matrix. The epoxy resin matrix softens when subjected to elevated temperatures with a resultant loss in strength of the composite. General purpose heat cured epoxy resins are only suitable for frac or bridge plugs operating in the lower 250 F and 8,000 kpsi service range. Higher performing tetra-functional epoxy resins can be used in the 350 F and 10,000 psi range provided the frac or bridge plug is designed with enough material to handle the stresses. The well completion industry would like to operate deeper fracking processes at higher temperatures and pressures but the heat distortion temperature for the epoxy resin has been a limitation to date.
The options for current commercially available resin systems to be used in downhole tools are limited. There are several resin systems that are known to have higher elevated temperature properties than the best epoxies, such as BMI (bismaleimide) or Cynate Ester resins. However, while these resins have high Tg (glass transition temperatures), they are not widely accepted in the frac and/or bridge plug market due to hydrothermal degradation and high costs. Additionally, retention of mechanical properties is not the only requirement for a resin system suitable for frac and/or bridge plugs. The resin must also have a mixed viscosity that is low enough to be processed by conventional frac plug manufacturing methods (for example, e.g., filament winding or convolute cloth wrap) and have sufficient pot-life for such processing. Consequently tetra-functional epoxies have been the primary resin system used to date for composite frac and bridge plugs.